1. Field of the Invention
The invention relates to a shift control apparatus and a shift control method for an automatic transmission, and particularly to a shift control that is performed when a multiple shift determination is made due to a determination for a second shift being made during a first shift.
2. Description of the Related Art
Automatic transmissions that shift among a plurality of speeds with different speed ratios by selectively engaging multiple friction coupling elements are widely used in vehicles, and so on. An automatic transmission switches from a first shift control to a second shift control when a multiple shift determination is made due to a determination for a second shift process being made during a first shift process, even if the first shift control is being executed. In this case, the automatic transmission performs a “return multiple shift” that is a shift for bringing the automatic transmission back to the speed that the automatic transmission was in before the first shift process was activated. Here, it is assumed that the second shift process determination that is described above is made before the inertia phase of the first shift process that the rotational speed of the input shaft of the automatic transmission changes, that is, when the rotational speed of the input shaft of the automatic transmission is still in the vicinity of the synchronous rotational speed for the speed to which the automatic transmission is to be shifted in the second shift process. In such a case, because the progress of the ongoing first shift process can not be accurately determined, if the second shift process is activated, it may cause the friction coupling elements to be engaged or released at incorrect timings and thus shift shocks may occur. That is, when the inertia phase of the first shift process has not yet started, it does not always mean that the friction coupling device that is released in the first shift process and engaged in the second shift process is still fully engaged. That is, that friction coupling element, before the beginning of the inertia phase of the first shift process, may be already in a fully released where no hydraulic fluid is left in the hydraulic cylinder or in a released state where a certain amount of hydraulic fluid is left in the hydraulic cylinder. Thus, in such a state, it is difficult to perform appropriate shift control of the automatic transmission. In the technology described in JP-A-2001-124193, the switching from the first shift process to the second shift process is carried out when it is determined that the first shift process has progressed sufficiently and thus the second shift process may be appropriately performed.
However, if the start of the second shift process is thus delayed, it diminishes the responses to the acceleration requests, etc, from the driver, and therefore discomfort during driving occurs.